Olefins have been traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefins such as ethylene and propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. For example, methanol, the preferred alcohol for light olefin production, may be converted to primarily ethylene and propylene in the presence of a molecular sieve catalyst. This process is referred to as a methanol-to-olefin (MTO) reaction process, which occurs in an MTO reaction system.
There are numerous technologies available for producing methanol including fermentation or the reaction of synthesis gas (syngas) derived from a hydrocarbon feed stream, which may include natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. Methanol is typically synthesized from the catalytic reaction of syngas in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor. Syngas is defined as a gas comprising primarily carbon monoxide (CO), hydrogen (H2) and preferably carbon dioxide (CO2). Optionally, syngas may also include unreacted feedstocks such as methane (CH4), ethane, propane or heavier hydrocarbons. Generally, the production of syngas involves a reforming reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Syngas production processes are well known, and include conventional steam reforming, autothermal reforming, or a combination thereof.
In an effort to increase energy and capital cost savings as well as olefin yield, U.S. Pat. No. 5,714,662 to Vora et al., the entirety of which is incorporated herein by reference, discloses integrating a methanol synthesis system with an MTO reaction system. Specifically, the Vora patent discloses a process for the production of light olefins from a hydrocarbon gas stream by a combination of reforming, oxygenate production, and oxygenate conversion wherein a crude methanol stream—produced in the production of oxygenates and comprising methanol, light ends, and heavier alcohols—is passed directly to the oxygenate conversion zone for the production of light olefins. The fusel oil in the crude methanol, which typically includes higher alcohols and is generally burned as a fuel in the methanol plant, is passed to the oxygenate conversion process for the additional production of light olefins. The Vora patent indicates that in so doing, the yield of ethylene, propylene, and butylenes can be enhanced at significant capital and operating cost savings by not requiring a complex and expensive distillation train for the production of high purity methanol.
A typical MTO reaction system may include an oxygenate/MTO byproduct separation system, which is adapted to separate unreacted oxygenates from MTO byproducts such as water. A typical methanol synthesis system includes a light ends separation system for separating light ends, e.g., unreacted syngas components, and product methanol from water, caustic salts, and fusel oil, e.g., C1–C4 alcohols and water, which are byproducts of the methanol synthesis process. Each of these separation systems may include one or more expensive and space-consuming separation units, e.g., distillation columns, pumps and heat exchangers. Thus, the need exists for reducing the number of these separation units.